The present invention relates to a luminescence device having a plurality of organic EL (Electroluminescence) elements in which at least a charge injection layer and a light-emitting layer are interposed between a pair of electrodes.
Since an electroluminescence element (hereinafter referred to as an xe2x80x9cEL elementxe2x80x9d) using electroluminescence causes luminescence, the visibility is high and the element is a completely solid state element so that it is excellent in the impact durability. Accordingly, its use as a luminescence element in various display devices has attracted much interest.
The EL elements include an inorganic EL element obtained by using an inorganic compound as a luminous material and an organic EL element obtained by using an organic compound as a luminous material. Especially in the organic EL element of these, the driving voltage is quite low as compared with the inorganic EL element, and the element can further easily be downsized. Accordingly, studies and developments to achieve practical use have been increasingly conducted.
In order to achieve the organic EL element to practical use, the increase in efficiency of the element performance and the improvement in the life time are indispensable. For solving these problems, the improvements in the luminous material and the device structure have been carried out.
As the organic EL element, an element based on a multilayered structure of anode/organic luminous layer/cathode and provided on this with a hole injection transport layer or an electron injection transport layer as a charge injection layer, as required, for example, an element having a construction of anode/hole injection transport layer/organic luminous layer/cathode, an element having a construction of anode/hole injection transport layer/organic luminous layer/electron injection transport layer/cathode or the like is known.
The hole injection transport layer here has a function of injecting holes efficiently from an anode and transporting the holes to the luminous layer, and it is, in many cases, constructed of a hole injection layer and a hole transporting layer.
Further, the electron injection transport layer has a function of injecting electrons efficiently from a cathode at good efficiency and transporting the electrons to the luminous layer. The light-emitting layer has a function of emitting light by recombination of holes and electrons injected. By the way, this electron injection transport layer is sometimes constructed of an electron injection layer and an electron transport layer.
It is considered that such an organic EL element can also be applied to various display devices. In constructing a display device with an organic EL element, it is considered to employ a dot matrix panel in which a plurality of organic EL elements are capable of simple matrix driving.
At this time, as a method for producing a display device with the use of organic EL elements, a method can be employed in which a substrate which is made of glass and on which surface transparent anode electrodes formed of an ITO (Indium Tin Oxide) film are patterned in striped shape is used, predetermined materials are vacuum-deposited in order on the surface of this substrate to form a plurality of organic EL elements containing at least a charge injection layer and a light-emitting layer, and a metal is deposited thereon to form cathode electrodes.
Incidentally, in this method, the respective layers constituting the organic EL element, such as the charge injection layer, the luminous layer and the like, are generally deposited uniformly on the surface of the above-described substrate.
In the display device produced by such a method, the edge of the transparent electrode formed on the substrate, for example, the ITO electrode, is steep, so that the thickness of the film formed on the side surface of the ITO electrode tends to be smaller than that of the other portion and a defect of electrical shortcircuiting sometimes occurs.
Further, in a display device having a display ability with higher resolution, total extension of the ITO electrode formed on the substrate is increased, and the length of the steep edge is increased, with the result that possibility of causing a defect of shortcircuiting is increased.
For this reason, when a hole injection layer as a charge injection layer is formed, it is a common practice that the film thickness of the hole injection layer is increased and the ITO electrode is completely covered with the hole injection layer to conceal the edge of the ITO electrode with the hole injection layer whereby the defect of shortcircuiting does not occur even though the edge of the ITO electrode is steep. This is because a conductivity of a hole injection layer is usually so high that the increase in the driving voltage is controlled to be relatively moderate when increasing the film thickness.
Meanwhile, in order to realize an organic EL element with high performance, it is indispensable to reduce the dissipation power thereof, and to this end, the decrease in the driving voltage is attempted. For example, in Japanese Patent Laid-Open No. 145192/1992 and Japanese Patent Laid-Open No. 45479/1997, it is proposed that the driving voltage is decreased by employing a conductive polymer having a high conductivity as a material of a hole injection layer formed on an organic EL element.
However, when a conductive polymer having a high conductivity is employed as a material of a hole injection layer, a film thickness of a hole injection layer is increased in the above-described display device. Accordingly, a DC resistance between a plurality of organic EL elements adjoined is decreased. There is a problem that when a voltage is applied to one organic EL element, a leakage current is passed through another organic EL element, and an unselected organic EL element is also ignited.
It is an object of the present invention to provide a luminescence device in which ignition of, among a plurality of organic EL elements provided, those other than the selected is prevented without impairing the decrease in the driving voltage.
The present invention is a luminescence device in which plural sets of anodes and cathodes are arranged opposite to each other and an charge injection layer and a light-emitting layer are formed between these anodes and cathodes whereby a plurality of organic EL elements can be emissive independently, characterized in that the conductivity a of the charge injection layer is set such that a leakage current from the organic EL element is 1/100 or less of a current passed through the organic EL element. In particular the conductivity a of the charge injection layer is set such that a leakage current from the organic EL element to the adjacent organic EL elements 1/100 or less of the current passed through the organic EL element.
In the present invention, the conductivity is set such that the leakage current from one organic EL element to another organic EL element(adjacent organic EL elements) among a plurality of the organic EL elements is satisfactorily small. Consequently, even though a voltage is applied to one selected organic EL element, an unselected organic EL element is not ignited.
Further, when the conductivity is set, as required, according to a size of electrodes constituting an anode and a cathode connected to an organic EL element, a gap of electrodes and the like, an organic EL element can be driven at low voltage even though employing such a conductivity that a leakage current from the organic EL element is 1/100 or less of a current passed through the organic EL element. Thus, the decrease in the driving voltage is not impaired.